Active clamp circuit

ABSTRACT

An active clamp circuit includes an active clamp switch having a drain node and a source node, an active clamp capacitor coupled in a series combination with the active clamp switch, a delay circuit, and an active clamp controller circuit coupled to the active clamp switch and to the delay circuit. The active clamp controller circuit is configured to i) receive an active clamp switch voltage based on a voltage developed across the drain node and the source node of the active clamp switch, ii) enable the active clamp switch based on a voltage amplitude of the active clamp switch voltage, and iii) disable the active clamp switch based on a delay signal generated by the delay circuit.

RELATED APPLICATION

This application is related to U.S. patent application Ser. No.16/145,819, filed Sep. 28, 2018, and entitled “Integrated Self-DrivenActive Clamp,” all of which is incorporated by reference herein in itsentirety.

BACKGROUND

Switch-mode power supplies (SMPS) are power management components inmodern electronic devices. They provide, among other things, efficientand galvanically isolated power to multiple loads. To achieve high powerprocessing efficiency and/or galvanic isolation, conventionally one ormore magnetically coupled elements, semiconductor switches andassociated gate driver circuits are required.

The magnetically coupled elements often suffer from non-trivial leakageinductance phenomena, which necessitate the need for affordable voltagesnubber circuits to control the semiconductor switch peakdrain-to-source voltages. Because of the price-sensitive nature of SMPS,the snubber circuits are conventionally limited to the cost-effectivepassive and power lossy resistor-capacitor-diode (RCD) configurations.

SUMMARY

In some embodiments, an active clamp circuit includes an active clampswitch having a drain node and a source node, an active clamp capacitorcoupled in a series combination with the active clamp switch, a delaycircuit, and an active clamp controller circuit. The active clampcontroller circuit is coupled to the active clamp switch and to thedelay circuit. The active clamp controller circuit is configured to i)receive an active clamp switch voltage based on a voltage developedacross the drain node and the source node of the active clamp switch,ii) enable the active clamp switch based on a voltage amplitude of theactive clamp switch voltage, and iii) disable the active clamp switchbased on a delay signal generated by the delay circuit.

In some embodiments, a power converter includes a transformer having aprimary winding and a secondary winding. A first winding node of theprimary winding is configured to be coupled to a voltage source toreceive an input voltage. The secondary winding is configured to becoupled to a load to provide an output voltage from the input voltage.The power converter includes an active clamp circuit and a main switchcoupled to a second winding node of the primary winding to control acurrent through the primary winding. The active clamp circuit includesan active clamp switch having a drain node and a source node, an activeclamp capacitor coupled in a series circuit combination with the activeclamp switch, a delay circuit, and an active clamp controller circuit.The active clamp controller circuit is coupled to the active clampswitch and to the delay circuit. The active clamp controller circuit isconfigured to i) receive an active clamp switch voltage based on avoltage developed across the drain node and the source node of theactive clamp switch, ii) enable the active clamp switch based on avoltage amplitude of the active clamp switch voltage, and iii) disablethe active clamp switch based on a delay signal generated by the delaycircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified circuit schematic of a conventional powerconverter.

FIG. 2 is a simplified circuit schematic of a power converter, inaccordance with some embodiments.

FIG. 3 is a simplified circuit schematic of a low-cost self-drivenactive clamp circuit, e.g., for use in the power converter shown in FIG.2, in accordance with some embodiments.

FIG. 4 shows simplified plots of signals related to the low-costself-driven active clamp circuit shown in FIG. 3, in accordance withsome embodiments.

FIG. 5-6 are portions of a process for clamping a voltage of a mainswitch of the power converter shown in FIG. 2 using the self-drivenactive clamp circuit shown in FIG. 3, in accordance with someembodiments.

DETAILED DESCRIPTION

Some embodiments described herein provide a low-cost self-driven activeclamp circuit and self-driven active clamping methods for use in a powerconverter that converts an input voltage to an output voltage using atransformer. In some embodiments, the self-driven active clamp describedherein replaces a diode of a resistor-capacitor-diode (RCD) snubbercircuit of a conventional power converter, advantageously converting theconventional power converter into a power converter having self-drivenactive clamping without needing to change additional control circuits ofthe power converter (such as a primary-side power management integratedcircuit, or a secondary side synchronous switch controller integratedcircuit). In other embodiments, the self-driven active clamp circuit isintegrated into an initial design of a power converter. In eitherembodiment, control of the power converter is advantageously simplifiedas compared to conventional solutions.

In systems sensitive to power losses and heat generation, thedissipation in lossy components in the form of heat is unsuitable. Thus,recycling of energy using an active clamping configuration within thesystem provides an opportunity for system form-factor reduction andpower efficiency improvement. Additionally, clamping the maximumdrain-source voltages of switching power transistors allows forincreased device reliability and use of power transistors with improvedfigure-of-merit (FOM). The improved FOM enables the SMPS to operate athigher switching frequency while maintaining high power processingefficiency. Furthermore, clamping the maximum drain-source voltages ofswitching power transistors allows for a reduction of the SMPS reactivecomponent size and cost.

The self-driven active clamp circuit, as compared to an RCD snubbercircuit, advantageously increases power processing efficiency of a powerconverter by recycling energy stored in a leakage inductance of thetransformer. In accordance with some embodiments, the self-driven activeclamp circuit clamps a primary side peak voltage of a main switch of thepower converter, which enables the power converter to utilize primaryside and/or secondary side switches having a lower voltage rating,leading to reduced power losses during switch conduction and/orswitching.

In some embodiments, the self-driven active clamp circuit enables anactive clamp switch based on a comparison between a reference voltage toa voltage, or an attenuated representation of the voltage, developedacross a drain node and a source node of the active clamp switch. Insuch embodiments, the self-driven active clamp circuit disables theactive clamp switch at the expiration of a delay. The delay is initiatedin response to the active clamp switch being enabled. In suchembodiments, the active clamp circuit advantageously can be implementedusing low-cost voltage comparison amplifiers. Additionally, an on-timeof the self-driven active clamp circuit can be advantageously reduced ascompared to conventional active clamp solutions because the active clampswitching does not need to coincide with a main switch turn-on time. Asa result, a smaller active clamp capacitor can be utilized as comparedto that used in conventional clamping circuits because the active clampcircuit resonant period is shorter. Because the active clamp capacitoris a high voltage component, using a smaller capacitance can result insignificant cost reduction benefits.

FIG. 1 is a simplified circuit schematic of a conventional powerconverter (“converter”) 100. Some elements of the converter 100 havebeen omitted from FIG. 1 to simplify the description of converter 100but are understood to be present. A voltage source V_(in)′ is receivedat the converter 100. V_(in)′ can be provided either as an alternatingcurrent (AC) or direct current (DC). An input side of the converter 100generally includes an input voltage filter block 122, a rectifier block116 (in the case of AC input), an input voltage buffer capacitor C1, anRCD snubber circuit block 114 (which includes a capacitor C2, a resistorR1 and a diode D1), a main switch M1′ driven by a pulse-width-modulation(PWM) signal PWM_(M1′), and a main switch controller circuit(“controller”) 118. The input voltage filter block 122, rectifier block116 and the input buffer capacitor C1 provide a filtered, buffered,rectified, or otherwise conditioned input voltage V_(in) to atransformer 102.

The transformer 102 transfers power from the input side of the converter100 to an output side of the converter 100 and generally includes aprimary winding 104 with a first node 108 and a second node 110, and asecondary winding 106. The output side of the converter 100 generallyincludes an output buffer circuit 112, a synchronous rectifier switchM2′, a synchronous rectifier switch controller circuit (“controller”)120, and a load R_(L)′.

The first node 108 receives V_(in). The second node 110 is coupled to adrain node of the main switch M1′. The main switch M1′ controls acurrent through the primary winding 104 to charge a magnetizinginductance L_(M) 105 of the transformer 102 during a first portion of aswitching cycle of the converter 100. The synchronous rectifier switchM2′ controls a current flow through the secondary winding 106 todischarge the transformer 102 into output buffer circuit 112 and theload during a subsequent portion of the switching cycle.

When the main switch M1′ is enabled by the controller 118 during thefirst portion of a switching cycle, current flows through the primarywinding 104 to a voltage bias node such as ground. The current flowthrough the primary winding 104 causes energy to be stored in themagnetization inductance L_(M) 105 and a leakage inductance L_(L) (notshown) of the transformer 102. When the main switch M1′ is disabled in asubsequent portion of the switching cycle, output voltage V_(out) isgenerated at the output buffer circuit 112 and is provided to the loadR_(L)′. When the main switch M1′ is turned off, a reflected voltage(nV_(out)) is developed at a drain node of the main switch M1′ at thesecond node 110. The contribution of the reflected voltage nV_(out) to adrain-source voltage V_(dsM1) of the main switch M1′ at the second node110 is expressed as:V _(dsM1) =V _(in) +nV _(out)  (Equation 1)where n is a turns ratio of the transformer 102. Energy stored in theleakage inductance L_(L) of the transformer 102 also contributes to thevoltage V_(dsM1) developed at the second node 110.

The RCD snubber circuit 114 prevents the voltage V_(dsM1) fromincreasing to a level that damages the main switch M1′. As V_(dsM1)rises, the diode D1 becomes forward biased and current flows into thecapacitor C2 and into the resistor R1 to dissipate energy, therebyclamping V_(dsM1) to a level that is within a safe operating range ofthe main switch M1′. However, in systems sensitive to power losses andheat generation, the dissipation in lossy components (e.g., the resistorR1) in the form of heat is unsuitable. To further increase theefficiency of the converter 100, the diode D1 can be replaced with anactively driven clamp switch driven by an active clamp drive signal.However, conventional active clamping circuits require a control signalor other means of synchronization from the controller 118. Thus, aconverter 100 that uses a controller 118 that is not already configuredto support conventional active clamping cannot easily be modified toimplement active clamping.

FIG. 2 is a simplified circuit schematic of a power converter(“converter”) 200 with a self-driven active clamp circuit 214, inaccordance with some embodiments. Some elements of the converter 200have been omitted from FIG. 2 to simplify the description of theconverter 200 but are understood to be present. Some elements of theconverter 200 are similar to elements of the converter 100 (e.g., themain switch M1 is similar to the main switch M1′). However, in theembodiment shown, all, or a portion (e.g., the diode D1), of the snubbercircuit 114 of the converter 100 has been replaced with the self-drivenactive clamp circuit (“active clamp circuit”) 214. In some embodiments,the resistor R1 of the conventional RCD 114 can advantageously beomitted from the active clamp circuit 214, providing further costsavings and improvements in power efficiency. In general, the converter200 includes an input side configured to receive an input voltage, andan output side configured to provide an output voltage from the inputvoltage, the input side being coupled to the output side by atransformer 202. The transformer 202 transfers power from the input sideof the converter 200 to the output side of the converter 200 andgenerally includes a primary winding 204 and a secondary winding 206.The primary winding 204 includes a first winding node 208 and a secondwinding node 210. The input side of the converter 200 generally includesan input filter block 222, a rectifier block 216, an input voltagebuffer capacitor C1, a main switch M1, a self-driven active clampcircuit (“active clamp circuit”) 214, and a main switch controller 218.A magnetizing inductance L_(M) of the transformer 202 is illustrated asa winding 205. Similar to that as was described with reference to thenode 108 and the node 110 of FIG. 1, the node 208 receives an inputvoltage V_(in) and a node 210 receives a drain-source voltage V_(dsM1)of the main switch M1.

The output side of the converter 200 generally includes an output buffer212, a synchronous rectifier switch M2, a synchronous rectifier switchcontroller circuit (“controller”) 220, and a load R_(L). As shown, theself-driven active clamp circuit 214 is connected between the node 208and the node 210. In some embodiments, one or both of the main switch M1and/or the synchronous rectifier switch M2 are field-effect transistors(FETs), each having a drain node, a source node, and a gate node tocontrol a conduction of current between the drain node and the sourcenode. In other embodiments, the synchronous rectifier switch M2 isreplaced with a diode.

Advantageously, in some embodiments, the active clamp circuit 214 canreplace the snubber circuit 114 of the converter 100 without makingsignificant modifications to the converter 100 (e.g., it does notrequire a control signal or other synchronization signal from thecontrollers 218, 220). Thus, in such embodiments, a converter 100 thatwas manufactured with a snubber circuit similar to the snubber circuit114 can be modified with the self-driven active clamp circuit 214 toperform active clamping. For example, in some embodiments, the diode D1of the conventional snubber circuit 114 can be replaced with theself-driven active clamp circuit 214. In other embodiments, theconverter 200 can be designed and or manufactured to include theself-driven active clamp circuit 214. In such embodiments, control ofthe converter 200 is advantageously simplified as compared toconventional solutions

FIG. 3 is a simplified circuit schematic of the self-driven active clampcircuit 214 of the converter 200 introduced with reference to FIG. 2, inaccordance with some embodiments. Some elements of the self-drivenactive clamp circuit 214 have been omitted from FIG. 3 to simplify thedescription of the self-driven active clamp circuit 214 but areunderstood to be present.

The active clamp circuit 214 prevents the voltage V_(dsM1) fromincreasing to a level that damages the main switch M1. The active clampcircuit 214 generally includes an active clamp capacitor C3, an activeclamp switch M3, a delay circuit 302, an active clamp controller circuit304, a gate driver circuit 306, and a voltage divider circuit 313. Thedelay circuit 302 generally includes resistors R4, R5, R6, capacitor C4,and a voltage comparison circuit 310. The active clamp controllercircuit 304 generally includes a voltage comparison circuit 316 and alogic circuit 318. In some embodiments, the active clamp controllercircuit 304 includes a reference voltage source 312 (e.g., a bandgap).In other embodiments, the reference voltage source 312 is outside of theactive clamp controller circuit 304. In the embodiment shown, thevoltage divider circuit 313 includes resistors R2, R3. In otherembodiments, the voltage divider circuit 313 includes other, oradditional, circuit components suitable for generating an attenuatedvoltage based on an input voltage. The active clamp switch M3 includes abody-diode, a drain node (‘D’), a source node (‘S’), and a gate node(‘G’) (i.e., a switch control node).

The active clamp capacitor C3 is connected in a series circuitcombination with the active clamp switch M3. The active clamp controllercircuit 304 is coupled to the gate node G of the active clamp switch M3through the gate driver circuit 306. The gate driver circuit 306 iscoupled to a bias voltage Vcc and to the node 210 to receive a railvoltage. The active clamp controller circuit 304 is configured toreceive an active clamp switch voltage (V_(ac)) 307 at node 308. Theactive clamp switch voltage 307 is based on a voltage developed acrossthe drain node D and the source node S of the active clamp switch M3(V_(dsM3)). The active clamp controller circuit 304 enables (i.e., turnson) the active clamp switch M3 based on a voltage amplitude of theactive clamp switch voltage 307 and disables (i.e., turns off) theactive clamp switch M3 based on a delay signal 305 generated by thedelay circuit 302 at the expiration of a time delay. The time delayprovided by the delay circuit 302 is initiated in response to the activeclamp switch M3 being enabled and expires after a duration of the delayhas elapsed. Upon receiving the delay signal 305, the active clampcontroller circuit 304 disables the active clamp switch M3.

The active clamp switch voltage (V_(ac)) 307 is generated by the voltagedivider circuit 313 that is directly connected across the drain node Dand the source node S of the active clamp switch M3 to receive thedrain-source voltage V_(dsM3) of the active clamp switch M3. In someembodiments, the voltage divider circuit 313 includes a seriescombination of the resistors R2 and R3. In such embodiments, a firstterminal of the resistor R2 is directly connected to the drain node D ofthe active clamp switch M3, a second terminal of the resistor R2 iscoupled to a first terminal of the resistor R3, and a second terminal ofthe resistor R3 is directly connected to the source node S of the activeclamp switch M3.

The reference voltage source 312 generates a reference voltage 314. Thevoltage comparison circuit (e.g., a comparator) 316 receives the activeclamp switch voltage 307 from the node 308 at a negative terminal (‘−’)and receives the reference voltage 314 at a positive terminal (‘+’). Thevoltage comparison circuit 316 generates a voltage comparison signal(V_(cmp)) 317 based on a comparison of the active clamp switch voltage307 and the reference voltage 314. In such embodiments, the active clampswitch M3 is enabled when the active clamp switch voltage 307 is lessthan the reference voltage 314.

The logic circuit 318 includes a set-reset (SR) latch circuit. A SETterminal of the of the SR latch circuit of the logic circuit 318 isconfigured to receive the voltage comparison signal 317. In response toreceiving the positive edge of the voltage comparison signal 317 at theSET terminal, the logic circuit 318 emits a pulse-width-modulation (PWM)signal 319 (i.e., an active clamp switch control signal PWM_(M3)) at afirst level (e.g., an asserted level) to enable the active clamp switchM3.

During light-load operation of the converter 200, the drain-sourcevoltage can be several hundred volts less than during non light-loadoperation. Advantageously, in addition to acting as a voltage divider,the series combination of the resistors R2, R3 implements a low-powermode for the active clamp circuit 214. That is, the drain-source voltageV_(dsM3) has to rise to a large enough level before the voltagecomparison signal 317 at the SET terminal is asserted.

A RESET terminal of the SR latch circuit of the logic circuit 318 isconfigured to receive the delay signal 305. In response to receiving thedelay signal 305 at the RESET terminal, the logic circuit 318 emits thePWM_(M3) signal 319 at a second level (e.g., a de-asserted level) todisable the active clamp switch M3. In addition, the logic circuit 318is configured to receive a power-on-reset (POR) signal at a POR terminalfrom a node 320 and is configured to receive anover-temperature-protection (OTP) signal at an OTP terminal from a node322. The logic circuit 318 is configured to disable the active clampswitch M3 in response to receiving either of the POR or the OTP signal.In some embodiments, the POR signal is generated by a POR circuit (notshown) and the OTP signal is generated by an OTP circuit (not shown).Circuits configured to generate POR and OTP signals are understood byone of ordinary skill in the art.

The delay circuit 302 includes a resistor divider circuit of the seriesconnected resistors R4, R5, configured to receive the bias voltage Vccat a first terminal and a voltage Vs from the source node of the activeclamp switch M3 at a second terminal. The resistor divider circuit R4,R5 generates a delay threshold voltage (V_(thr)) 309 which is receivedat a negative terminal (‘−’) of the voltage comparison circuit 310. Aresistor-capacitor (RC) circuit of the capacitor C4 and the resistor R5receives the PWM_(M3) signal 319 and generates a ramp signal (V_(RC))311 as the capacitor C4 is charged by the PWM_(M3) signal 319. Thevoltage comparison circuit 310 receives the ramp signal 311 at apositive terminal (‘+’) and compares the ramp signal 311 to the delaythreshold voltage 309. When the ramp signal 311 is equal to or greaterthan the delay threshold voltage 309, the voltage comparison circuit 310emits the delay signal 305. Thus, a duration of the delay provided bythe delay circuit 302 is configured based on a choice of values of theresistors R4, R5, R6 and of the capacitor C4. The delay signal 305 isreceived at the RESET terminal of the logic circuit 318, and inresponse, the logic circuit 318 disables the active clamp switch M3 byemitting the PWM_(M3) signal 319 at the second level as previouslydescribed.

The gate driver circuit 306 drives (i.e., enables and disables) theactive clamp switch M3. In some embodiments, the active clamp switch M3is a current-bidirectional two-quadrant switch. The gate node G of theactive clamp switch M3 controls a conduction of current between thedrain node D and the source node S. The drain node D and the source nodeS of the active clamp switch M3 are in a series circuit combination withthe active clamp capacitor C3. In some embodiments, the active clampswitch M3 includes a diode, other than a body-diode, which is configuredto pass a current between the source node S and the drain node D (in afirst current direction) when the diode is forward biased (e.g., whensufficient voltage is developed across the source and drain of theactive clamp switch M3). When the gate node G of the active clamp switchM3 is driven by the switch control signal, the active clamp switch M3passes current bidirectionally (e.g., in the first current direction,and/or a second current direction). In the first current direction,current flows from the primary winding 104, through the active clampswitch M3, and into the active clamp capacitor C3. In the second currentdirection, current flows from the active clamp capacitor C3, through theactive clamp switch M3, and into the primary winding 204.

During a portion of the switching cycle when the main switch M1 andactive clamp switch M3 are both off, the active clamp switch currenti_(sd) flows from the primary winding 204, through the body-diode of theactive clamp switch M3, to the active clamp capacitor C3. During asubsequent portion of the switching cycle when the main switch M1 is offand the active clamp switch M3 is on, the current i_(sd) oscillatesbetween the active clamp capacitor C3, the magnetizing inductance L_(M),and other intended or parasitic reactive elements of the converter 200.

FIG. 4 provides simplified example plots 402, 404, 406, 408, and 410which illustrate a relationship between signals of the converter 200,including those of the active clamp circuit 214, across a window oftime, in accordance with some embodiments. The plot 402 illustrates thefull-range voltage drain-source voltage (V_(dsM1)) of the main switch M1across the window of time. The plot 404 show the active clamp switchvoltage (V_(ac)) 307 at node 308. As the voltage V_(dsM1) at the drainnode of the main switch M1 rises, the active clamp switch voltage(V_(ac)) will correspondingly fall (i.e., as measured relative to thedrain node D of the active clamp switch M3). When the active clampswitch voltage (V_(ac)) 307 falls to a voltage level that is less thanthe reference voltage 314 (shown as a dashed line V_(REF)), the voltagecomparison circuit 316 asserts a rising edge on the SET input of the SRlatch circuit of the logic circuit 318, as shown in the plot 406. Inresponse, the logic circuit 318 emits an asserted PWM_(M3) signal, asshown in the plot 410, thereby enabling the active clamp switch M3 andclamping (i.e., limiting) the voltage V_(dsM1) at the node 210 to amaximum voltage that is within a safe operating range of the main switchM1. For example, in some embodiments, the maximum voltage is a voltagethat is less than a maximum operating voltage of the main switch M1.

The asserted PWM_(M3) signal also initiates a time delay of the delaycircuit 302. The plot 408 shows the ramp signal (V_(RC)) 311 rising inresponse to the asserted PWM_(M3) signal. When the ramp signal (V_(RC))311 is equal to or greater than the delay threshold voltage (V) 309, thedelay circuit 302 asserts a rising edge on the RESET input of the SRlatch circuit of the logic circuit 318, as shown in the plot 406. Inresponse, the logic circuit 318 emits a de-asserted PWM_(M3) signal, asshown in the plot 410, thereby disabling the active clamp switch M3.

FIG. 5 is a portion of an example process 500 for clamping a voltage ofa main switch of a power converter (e.g., the converter 200), inaccordance with some embodiments. The particular steps, order of steps,and combination of steps are shown for illustrative and explanatorypurposes only. Other embodiments can implement different particularsteps, orders of steps, and combinations of steps to achieve similarfunctions or results. At step 502, an input voltage is received at aprimary winding (204) of a transformer (202) of the power converter(200). At step 504, a current through the primary winding (204) iscontrolled using a main switch (M1) of the power converter (200). Atstep 506, a voltage of the main switch (i.e., at the second node 210) isclamped to a maximum voltage using an active clamp switch (M3) of anactive clamp circuit (214). The active clamp switch (M3) is enabledbased on an active clamp switch voltage (V_(ac)) developed across theactive clamp switch (M3), and the active clamp switch (M3) is disabledbased on a delay circuit (302).

Details of step 506 are presented in FIG. 6, in accordance with someembodiments. The particular steps, order of steps, and combination ofsteps are shown for illustrative and explanatory purposes only. Otherembodiments can implement different particular steps, orders of steps,and combinations of steps to achieve similar functions or results.

At step 602, the active clamp switch voltage (V_(ac)) is received at anactive clamp controller circuit (304) of the active clamp circuit (214).At step 604, it is determined if the active clamp switch voltage (307)is less than a reference voltage (314). If the active clamp switchvoltage (307) is not less than the reference voltage (314), flow returnsto step 602. Advantageously, if the active clamp switch M3 voltageV_(dsM3) never rises above the reference voltage (314) (e.g., duringlight-load-operation of the power converter 200), the process flow willremain at step 602. If, at step 604, it is determined that the activeclamp switch voltage (307) is less than the reference voltage (314),flow continues to step 606. At step 606, the active clamp switch (M3) isenabled. When the active clamp switch (M3) is enabled, the active clampcircuit (214) clamps (i.e., limits) a voltage of a main switch (M1) ofthe power converter (200). At step 608, in response to the active clampswitch (M3) being enabled, a delay at a delay circuit (302) of theactive clamp circuit 214 is initiated. At 610, it is determined if thedelay that was initiated at step 608 has expired. If the delay has notexpired, flow remains at step 610. If it is determined a step 610 thatthe delay has expired, flow continues to step 612. At step 612, theactive clamp switch (M3) is disabled. When the active clamp switch (M3)is disabled, the active clamp circuit (214) is no longer clamping avoltage of a main switch (M1) of the power converter (200).

Reference has been made in detail to embodiments of the disclosedinvention, one or more examples of which have been illustrated in theaccompanying figures. Each example has been provided by way ofexplanation of the present technology, not as a limitation of thepresent technology. In fact, while the specification has been describedin detail with respect to specific embodiments of the invention, it willbe appreciated that those skilled in the art, upon attaining anunderstanding of the foregoing, may readily conceive of alterations to,variations of, and equivalents to these embodiments. For instance,features illustrated or described as part of one embodiment may be usedwith another embodiment to yield a still further embodiment. Thus, it isintended that the present subject matter covers all such modificationsand variations within the scope of the appended claims and theirequivalents. These and other modifications and variations to the presentinvention may be practiced by those of ordinary skill in the art,without departing from the scope of the present invention, which is moreparticularly set forth in the appended claims. Furthermore, those ofordinary skill in the art will appreciate that the foregoing descriptionis by way of example only and is not intended to limit the invention.

What is claimed is:
 1. An active clamp circuit comprising: an activeclamp switch having a drain node and a source node; an active clampcapacitor coupled in a series combination with the active clamp switch;a delay circuit; and an active clamp controller circuit coupled to theactive clamp switch and to the delay circuit, the active clampcontroller circuit being configured to i) receive an active clamp switchvoltage based on a voltage developed across the drain node and thesource node of the active clamp switch, ii) enable the active clampswitch based on a voltage amplitude of the active clamp switch voltage,and iii) disable the active clamp switch based on a delay signalgenerated by the delay circuit; wherein the delay circuit comprises: aresistor divider circuit to receive a bias voltage and a voltage fromthe source node of the active clamp switch to generate a delay thresholdvoltage; a resistor-capacitor (RC) circuit to receive an active clampswitch control signal and the voltage from the source node of the activeclamp switch to generate a ramp signal in response to the active clampswitch control signal, the active clamp switch control signal beingconfigured to enable and disable the active clamp switch; and a firstvoltage comparison circuit configured to i) receive the delay thresholdvoltage, ii) receive the ramp signal, iii) compare the ramp signal tothe delay threshold voltage, and iv) disable the active clamp switchbased on a comparison of the ramp signal and the delay thresholdvoltage.
 2. The active clamp circuit of claim 1, wherein: a delay of thedelay circuit is initiated in response to the active clamp switch beingenabled and expires after a duration of the delay has elapsed, theactive clamp switch being disabled after the duration of the delay haselapsed.
 3. The active clamp circuit of claim 2, wherein: the durationof the delay is inversely proportional to a switching frequency of theactive clamp switch.
 4. The active clamp circuit of claim 1, furthercomprising: a voltage divider circuit directly connected across thedrain node and the source node of the active clamp switch; wherein: thevoltage divider circuit is configured to generate the active clampswitch voltage based on the voltage developed across the drain node andthe source node of the active clamp switch; and the active clampcontroller circuit is configured to receive the active clamp switchvoltage from the voltage divider circuit.
 5. The active clamp circuit ofclaim 4, wherein the active clamp controller circuit further comprises:a second voltage comparison circuit that is configured to i) receive theactive clamp switch voltage, ii) receive a reference voltage, and iii)generate a voltage comparison signal based on a comparison of the activeclamp switch voltage and the reference voltage; wherein: the activeclamp switch is enabled based on the voltage comparison signal.
 6. Theactive clamp circuit of claim 5, wherein the active clamp controllercircuit further comprises: a logic circuit comprising a SET-RESET (SR)latch circuit; wherein: a SET terminal of the SR latch circuit isconfigured to receive the voltage comparison signal, the SR latchcircuit being configured to enable the active clamp switch in responseto receiving, at the SET terminal, the voltage comparison signal; and aRESET terminal of the SR latch circuit is configured to receive thedelay signal generated by the delay circuit, the SR latch circuit beingconfigured to disable the active clamp switch in response to receiving,at the RESET terminal, the delay signal.
 7. The active clamp circuit ofclaim 1, wherein: the active clamp controller circuit is configured toreceive an over-temperature indication signal and to receive apower-on-reset signal; and the active clamp controller circuit isconfigured to disable the active clamp switch in response to receivingeither of the over-temperature indication signal or the power-on-resetsignal.
 8. The active clamp circuit of claim 1, wherein: the activeclamp switch is coupled to a node of another switch; and the activeclamp switch clamps a voltage at the node of the other switch to amaximum voltage.
 9. The active clamp circuit of claim 8, wherein: thenode of the other switch is coupled to a winding of a transformer; andthe other switch controls a current through the winding of thetransformer.
 10. A power converter comprising: a transformer having aprimary winding and a secondary winding, a first winding node of theprimary winding being configured to be coupled to a voltage source toreceive an input voltage, the secondary winding being configured to becoupled to a load to provide an output voltage from the input voltage; amain switch coupled to a second winding node of the primary winding tocontrol a current through the primary winding; and an active clampcircuit comprising: an active clamp switch having a drain node and asource node; an active clamp capacitor coupled in a series circuitcombination with the active clamp switch; a delay circuit; and an activeclamp controller circuit coupled to the active clamp switch and to thedelay circuit, the active clamp controller circuit being configured toi) receive an active clamp switch voltage based on a voltage developedacross the drain node and the source node of the active clamp switch,ii) enable the active clamp switch based on a voltage amplitude of theactive clamp switch voltage, and iii) disable the active clamp switchbased on a delay signal generated by the delay circuit; wherein thedelay circuit comprises: a resistor divider circuit to receive a biasvoltage and a voltage from the source node of the active clamp switch togenerate a delay threshold voltage; a resistor-capacitor (RC) circuit toreceive an active clamp switch control signal and the voltage from thesource node of the active clamp switch to generate a ramp signal, theactive clamp switch control signal being configured to enable anddisable the active clamp switch; and a first voltage comparison circuitconfigured to i) receive the delay threshold voltage, ii) receive theramp signal, and iii) generate the delay signal based on a comparison ofthe delay threshold voltage to the ramp signal; wherein: the activeclamp controller circuit is configured to disable the active clampswitch in response to receiving the delay signal.
 11. The powerconverter of claim 10, further comprising: a voltage divider circuitdirectly connected across the drain node and the source node of theactive clamp switch; wherein: the voltage divider circuit is configuredto generate the active clamp switch voltage based on the voltagedeveloped across the drain node and the source node of the active clampswitch; and the active clamp controller circuit is configured to receivethe active clamp switch voltage from the voltage divider circuit. 12.The power converter of claim 11, wherein the active clamp controllercircuit further comprises: a second voltage comparison circuit that isconfigured to i) receive the active clamp switch voltage, ii) receive areference voltage, and iii) generate a voltage comparison signal basedon a comparison of the active clamp switch voltage and the referencevoltage; wherein: the active clamp switch is enabled based on thevoltage comparison signal.
 13. The power converter of claim 12, whereinthe active clamp controller circuit further comprises: a SET-RESET (SR)latch circuit; wherein: a SET terminal of the SR latch circuit isconfigured to receive the voltage comparison signal, the SR latchcircuit being configured to enable the active clamp switch in responseto receiving the voltage comparison signal at the SET terminal; and aRESET terminal of the SR latch circuit is configured to receive thedelay signal generated by the delay circuit, the SR latch circuit beingconfigured to disable the active clamp switch in response to receivingthe delay signal at the RESET terminal.
 14. The power converter of claim10, wherein: the active clamp controller circuit is configured toreceive an over-temperature indication signal and to receive apower-on-reset signal; and the active clamp controller circuit isconfigured to disable the active clamp switch in response to receivingeither of the over-temperature indication signal or the power-on-resetsignal.
 15. The power converter of claim 10, further comprising: a mainswitch controller circuit that is communicatively coupled to the mainswitch to control a power transfer from the primary winding to thesecondary winding; wherein: the active clamp controller circuit iscommunicatively isolated from the main switch controller circuit. 16.The power converter of claim 10, wherein: the active clamp switch is acurrent-bidirectional two-quadrant switch; the source node and the drainnode of the active clamp switch are in the series circuit combinationwith the active clamp capacitor; and a switch control node of the activeclamp switch is coupled to the active clamp controller circuit tocontrol an active clamp switch current between the drain node and thesource node of the active clamp switch.
 17. The power converter of claim16, wherein: the active clamp switch is configured to pass the activeclamp switch current between the source node and the drain node inresponse to a voltage developed across the source node and the drainnode; and the active clamp switch is configured to pass the active clampswitch current between the drain node and the source node in response toa switch control signal received at the switch control node.